Many processes are designed with an electrical motor providing rotational power as an input source (i.e., prime mover) that delivers torque to an output. Equipment examples of possible outputs would be remote fans, pumps, compressors, conveyors, etc. In many cases, it is not necessary or desirable to drive the output at full speed. An example would be a fan that is sized for a worst-case condition, but at some operating conditions full air flow is not necessary for the process or equipment associated with the fan. This could also be true of pumps and compressors, where full flow is not needed at a given time.
There are several alternatives available to allow for controllability of motor/torque output. For example, a friction clutch can be employed to connect or disconnect the motor shaft to the output. This allows the output to be selectively turned on and off depending on the need of the process. Another alternative is to use a variable speed drive to control the torque input provided by the electric motor. This is typically in the form of a variable frequency drive (VFD). The VFD effectively changes the frequency of the AC power delivered to the electric motor. The motor speed is directly linked to the frequency of the AC power. VFD's are relatively complex to install and operate, particularly in a harsh environment. The VFD power electronics are sensitive to the heat and moisture making them difficult and expensive to protect in outdoor and harsh operating environments. In many cases, VFD's are not employed due to these complexities and limitations.
Friction clutches have been used for a long time to connect and disconnect rotating processes from their respective rotating input power sources. Friction clutches are a good choice when the process can benefit from being disconnected for substantial periods of time. When a process needs to be connected frequently or when a reduced (but non-zero) output speed would be beneficial, friction clutches are often not a preferred choice. This is due to the substantial wear on the friction material and mating surfaces that results in reliability concerns and relatively high maintenance costs.
Viscous friction clutches have been successfully deployed in automotive applications to drive cooling fans and pumps. Viscous clutches are desirable because they are able to control an output over a wide speed range. They are also desirable due to the use of a shearing fluid as the torque transfer means. The shearing fluid has a long service and life and is generally maintenance-free. However, stationary (i.e., non-rotating) mounting brackets used with many viscous clutches can undesirably add mass, occupy large amounts of space, and limit mounting locations for the associated clutch.
It is therefore desired to provide a viscous clutch with an alternative configuration that provides a modular control coil assembly and/or provides both input and output on a rear side of the clutch.